mmg_233_2013_genetics_genomicswikiaorg-20200214-history
Genetic Control of Cell Cycle Division
Saccromyces cerevisiae This model organism is a type of yeast that has been used in winemaking, baking and brewing since ancient times, but more recently it has become the most intensely studied eukaryotic organism. Saccromyces cervisiae exist in both haploid and diploid forms and reproduce through a process known as budding (Saccharomyces 2013.) The strength of this organism as a model lies in its ability to go through the same cell cycle in both the haplo and diplophase. Furthermore, the size of the "body" or bud coming off of the primary cell is a good indicator of the progress in the cell cycle. Given these characteristics, recessive mutations that impair the cell cycle to progress normally can be isolated in haploid cells and used to further study the issues that arise (Genetic 1973.) Genetic Control of Cell Cycle Division Hartwell et. al began their research by isolating strands of temperature sensitive S. cerevisiae mutants that had arrested between metaphase and anaphase of the cell division cycle. Two strands of yeasts had been treated with N-methyl-N'-nitro-N-nitrosoguanidinaes or ethylmethane sulfonate to induce mutagenesis and the permissive temperature was set at 23 degrees C and the restrictive temperature was set at 36 degrees C. At this point in time the cdc protein had been isoated in strands of yeast, but its purpose was not yet determined. The induced mutagenic compounds altered cell division and in some of the cells present there was a mutated form of the cdc gene. By combining these two ideas Hartwell began analyzing cells that exhibited an increased cell cycle rate or a complete block in the cell cycle and tried to determine if the cdc gene was causing this. Yeast strands were then to culture at the permissive and restrictive temperatures and they were viewed by time lapse photography to isolate cells that could be controlled by temperature. During this process 148 temperature sensitive cdc mutants were isolated that could be complementarily organized into 32 groups. These temperature sensitive yeasts exhibited deviations from normal cell cycle divions. Each one of the groups was analyzed and associated with a single gene and 14 of the genes were located on the yeast genetic map. It was determined that the arrested yeast cells were not properly forming the cdc complex and were therefore unable to go through the cell cycle at a normal rate. Cyclin b Hartwell did not yet realise that there was another protein in the complex that inhibited movement from metaphase to anaphase. This was discovered (accidentially) by Tim Hunt when he was trying to do a simple experiment that showed that protein concentrations were different before and after fertilization. (In contrast to Hartwell, Hunt did his experiments on sea urchin eggs.) Before a division cells build up proteins present within the cellular matrix so that both halves will be able to survive following the separation. Accordingly it is expected that protein levels should increase approaching a division. Interstingly lane A (See image to the right) showed a decrease in protein concentration before division. This meant that the protein found within this lane was actually being broken down and therefore it was likely that it had something to do with the division process. Tim Hunt spoke with Hartwell and Nurse and found that these results correlated with similar findings for another protein known as cdc2 in yeast. This lead to the discovery of the cdc2/cyclin b complex that is an inhibitor of the anaphase promoting complex (APC). (Cyclin, 1983) Today it is known that a calcium regulated pathway exists that involves of a kinase cascade of phosphorylations. Mos phosphorylates MEK which phosphorylates MAPK that then phosphorylates RSK which then finally phosphorylates Emi2. Emi2 inhibits the APC. When calcium is added to the system it binds to calcineurin which then inhibits the phosphorylation of MAPK and ends the cascade. This allows APC to be recruited and mark the cyclin b/ cdc2 complex with ubiquitin for degradation. This is also acheived when calcium recruits CamKII which then directly marks Emi2 for degradation via ubiquitin (5). Works Cited #Genetic Control of the Cell Division Cycle in Yeast: V. Genetic Analysis of cdc Mutants LELAND H. HARTWELL, ROBERT K. MORTIMER*,'''JOSEPH '''CuLOm 'AND '''MARILYN cuLo?TI 'Department ''of 'Genetics, University ''of Washington, Seattle, Washington ''98195 ''Manuscript received January 2, 1973 #http://www.bioinfo.org.cn/book/Great%20Experments/great21.htm #Saccharomyces cerevisiae . (2013, October 17). In Wikipedia, The Free Encyclopedia. Retrieved 16:56, November 6, 2013, ' # Cyclin: a protein specified by maternal mRNA in sea urchin eggs that is destroyed at each cleavage division. Evans T, Rosenthal ET, Youngblom J, Distel D, Hunt T. # A direct link of the Mos–MAPK pathway to Erp1/Emi2 in meiotic arrest of Xenopuslaevis eggs